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Area Forecast Discussion (AFD)

Issued by Eureka, CA (EKA)

FXUS66 KEKA 201143

National Weather Service Eureka CA
443 AM PDT Fri Jul 20 2018

.SYNOPSIS...Warm temperatures will continue through next week. In
addition, isolated thunderstorms will be possible this afternoon
through Sunday over Trinity and Mendocino Counties.


.DISCUSSION...The combination of an upper-level anticyclone over 
the southern High Plains, and a weak cyclone positioned offshore 
from Baja CA, are aiding in transporting subtropical moisture 
northward across CA. However, monsoonal moisture flux into
northwestern CA will temporarily be inhibited by an upper trough 
positioned over the PACNW. A midlevel deformation zone separating
dry air with the trough to the north, and moist air to the south,
will gradually be shunted northward this afternoon and night. 
This will allow PW values from 1.0-1.5 inches to enter Mendocino 
County late Friday night through Saturday...with moisture 
continuing to spread north across the region during the remainder 
of the weekend. 

The increase in moisture combined with steep mid/upper-level 
lapse rates, and modest isentropic upglide, may be sufficient for 
the development of an isolated elevated thunderstorm late tonight
over Mendocino County. However, the more probable scenario 
(albeit still fairly low) for thunderstorm development will occur 
Saturday and Sunday afternoon as strong boundary layer 
destabilization occurs over the mountains of northeastern 
Mendocino County and Trinity County.

Otherwise, warm midlevel temperatures (850 mb temperatures at or
above 23 C) and strong surface heating will favor highs in the
upper 90s to mid 100s across the valleys of Trinity, interior Del
Norte, and northeastern Mendocino Counties this afternoon. The
risk of heat related illness will subsequently become elevated,
which has prompted the issuance of another heat advisory.
Temperatures across the interior are forecast to cool slightly 
going into the weekend, but will likely warm back into the upper 
90s to low 100s during much of next week.


.AVIATION...Little in the way of low cloudiness has formed across
the area this morning. However, the inland thermal trough has 
shifted W overnight, particularly S of Cape Mendocino. As a 
result, a southerly flow reversal has begun off the Mendocino 
County coast, and coastal low clouds have begun to develop near 
and S of Fort Bragg. This cloudiness is expected to advect 
northward through the day as S flow becomes more pronounced. 
However, high resolution model data keeps low-level flow out of 
the N near and north of the Cape. As a result, although low clouds
will continue thickening S of the Cape, they are not expected to 
make their way N of the Cape through the afternoon. Low clouds may
advect N across the Russian River Valley early this morning and 
approach KUKI. However, ceilings are not expected at the airport. 
A few thunderstorms are possible over the Yolla Bollys this 
afternoon, but they are expected to remain well NE-E of KUKI. 
Winds across most of the land areas will be generally light, but N
winds will increase late this morning and this afternoon at KCEC 
with gusts to 25 knots. /SEC


.MARINE...N winds are peaking over the N outer waters early this 
morning. As mentioned, the surface thermal trough has begun 
pivoting across the S waters. As a result, winds will be 
decreasing across the area today and tonight, although winds will 
remain fairly strong in the N outer waters. Winds will become S-Sw
over the S inner waters as the surface flow reverses with a 
pronounced eddy forming S of Cape Mendocino. However, winds 
between Cape Mendocino and Shelter Cover will remain mostly out of
the N and gusty into the afternoon. Have cancelled the Hazardous 
Seas Warning for the N inner waters, although conditions near Cape
Mendocino will likely remain near criteria through the afternoon.
Winds will become light across all of our waters by late Sunday 
and into early next week, with northerlies increasing again by the
middle of next week. /SEC


.FIRE WEATHER...Hot and dry conditions will continue to affect 
much of the region today, and while some modest heat relief is 
expected over the weekend, temperatures are only forecast to fall
3 to 8 degrees area-wide. Thereafter, temperatures will likely 
become hot once again Monday through the remainder of next week.

Otherwise, northeasterly winds gusting to 30 mph are occurring
across the higher ridges of Del Norte County. Winds will likely
slacken during the hottest/driest part of Friday, and then re- 
strengthen Friday night as RH values experience moderate
recoveries. The expected temporal offset between the nocturnal 
peak in wind speeds, and low afternoon RH, continues to preclude 
the issuance of a red flag warning.

Elsewhere, monsoon moisture will spread north across Mendocino 
County late tonight, and will aid in a very slight chance for
elevated thunderstorm development. Thereafter, moisture will 
continue to spread north across NWRN CA during the weekend. 
Isolated thunderstorms across the interior mountains of Trinity 
and northeastern Mendocino Counties will be possible Saturday and 
Sunday afternoon/evening. /BRC/JMG


CA...Heat Advisory from noon today to 9 PM PDT this evening for 

     5 AM PDT early this morning for PZZ455.

     Small Craft Advisory until 4 AM PDT Saturday for PZZ450.

     Gale Warning until 4 AM PDT Saturday for PZZ470.

     Gale Warning until 3 PM PDT this afternoon for PZZ475.



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